Optical fibers, conventionally have glass-based filaments that have a core surrounded concentrically by one or more cladding layers having appropriate indices of refraction to guide light by total internal reflection. The outer cladding layer likewise is typically covered by an external layer. The optical fiber can be designed to be a single mode fiber or a multi-mode fiber.
Fiber designs have been proposed with hollow cores, sometimes referred to as “holey fibers” or “photonic crystal” fibers (PCF) or microstructured fiber. Known methods of making microstructured or PCF fibers include using stacked arrays of cylindrical tubing or capillaries and/or drilling longitudinal holes or bores into a fiber preform.
JP2005-338436 describes a hole assisted optical fiber which comprises a ring of holes in a cladding around a core. The size and number of holes is defined by formulae that define a maximum of about 61% hole: 39% glass for the amount of hole around a circle going through the holes.
In optical distribution frames widely used today, single mode fibers from the central office cable have to be patched to fibers going to the subscriber, i.e. optical network units (ONU). The fibers coming from the central office and from the subscriber are generally coming from below the ground, and therefore it has been necessary to provide a fiber loop inside the network hub when two parallel positioned fibers are to be connected or spliced in-line. The spare length of fiber required because of the in-line splicing configurations used until now, results in storage and management problems, increasing the size of the connector cabinets and the complexity of the tasks involved in making and changing connections.